This invention relates to pressure-sensitive adhesive compositions, and, more particularly, to pressure-sensitive adhesive compositions formed from at least two polymeric materials at least one of which is a pressure-sensitive adhesive, and to methods of making blended pressure-sensitive adhesives and to adhesive coated articles.
There is an ongoing need to modify pressure-sensitive adhesives to meet the needs of new applications. In general, when additives are incorporated into pressure-sensitive adhesives to modify their properties, care must be taken to avoid a loss in peel adhesion or shear strength. This has prevented a wide use of thermoplastic materials as modifiers.
Major classes of pressure-sensitive adhesives include tackified natural rubbers; synthetic rubbers such as butyl rubber; and tackified linear, radial, star, and branched and tapered styrene block copolymers, such as styrene-butadiene, styrene-ethylene/butylene and styrene-isoprene; polyurethanes; polyvinyl ethers; acrylics, especially those having long chain alkyl groups; poly-xcex1-olefins; and silicones.
Generally, when additives are used to alter properties of pressure-sensitive adhesives, the additives need to be miscible with the pressure-sensitive adhesive or to form homogeneous blends at the molecular level. Some types of pressure-sensitive adhesives have been modified with tackified thermoplastic elastomers, thermoplastics, and elastomers. For example, thermoplastic materials have been added to polymerized hot melt acrylic pressure-sensitive adhesives wherein the thermoplastic is a packaging material or recyclable tape backings. In these cases, the type and amount of thermoplastic material is controlled so that the thermoplastic material can function as a packaging material while avoiding degradation of the adhesive properties of the pressure-sensitive adhesive.
However, more often than not when a non-tacky thermoplastic additive is blended with a pressure-sensitive adhesive, reduction of the overall adhesive properties of the blend (as compared to the pressure-sensitive adhesive only) are observed. Thermoplastic polymers have been added to styrene block copolymer adhesives to reduce the tack of the resulting pressure-sensitive adhesives for application of protective sheets to large area surfaces.
Pressure-sensitive adhesives, whether modified or not have been used for more than half a century for a variety of purposes. Generally, pressure-sensitive adhesives are used in tapes wherein a tape comprises a backing, or substrate, and a pressure-sensitive adhesive. Typically, a pressure-sensitive adhesive adheres with no more than applied finger pressure and can be permanently tacky.
In the medical field, pressure-sensitive adhesive tapes are used for many different applications in the hospital and health areas. For most applications, tapes are applied directly to a patient""s skin. It is important that the pressure-sensitive adhesive tape be compliant and non-irritating to the skin, as well as adhering to the skin without causing damage to the skin when the tape or adhesive coated article is removed. A particularly useful medical application for pressure-sensitive adhesive tapes and articles is in the field of transdermal patches. Such patches can be used as drug transport membranes or to attach drug transport membranes to skin.
Although pressure-sensitive adhesive tapes and articles are widely used in the medical field, pressure-sensitive adhesive tapes and articles find widespread use in many other applications. For example, transfer tapes can be used to adhere two surfaces together such as the flaps of packing material or fabric to a surface. However, transfer tape adhesives generally have little tensile strength and one solution has been to add glass fibers to provide tensile strength.
Another use is in the field of labels, which require a large variety of pressure-sensitive adhesives due to a wide variety of surfaces. However, the pressure-sensitive adhesives must be able to be cut easily without stringing out or oozing to permit efficient manufacturing processes.
Pressure-sensitive adhesives require a delicate balance of viscous and elastic properties that result in a four-fold balance of adhesion, cohesion, stretchiness and elasticity. Pressure-sensitive adhesives generally comprise elastomers that are either inherently tacky, or elastomers or thermoplastic elastomers that are tackified with the addition of tackifying resins.
In one aspect, the present invention provides a pressure-sensitive adhesive comprising a blend of at least two components, wherein the first component is at least one pressure-sensitive adhesive and the second component is at least one thermoplastic material, wherein the components form a blended composition having more than one domain and, wherein one domain is substantially continuous (generally, the pressure-sensitive adhesive) and the other domain is substantially fibrillous to schistose (generally, the thermoplastic material).
Alternatively, the second component can be (a) at least one thermoplastic elastomer, as described in Ser. No. 08/578,010, filed Dec. 22, 1995 with a common assignee, (b) at least one elastomer with a tackifying resin as described in Ser. No. 08/577,603, filed Dec. 22, 1995 with a common assignee, or (c) at least one elastomer.
Advantageously, blended pressure-sensitive adhesives of the present invention provide adhesives having one or more of the following properties. These properties are improvements over a pressure-sensitive adhesive prior to blending it with a thermoplastic material. These properties include:
(1) a peel adhesion greater than and shear strength similar to that of the pressure-sensitive adhesive component if used alone,
(2) a shear strength greater than and peel adhesion similar to that of the pressure-sensitive adhesive component if used alone,
(3) an anisotropic peel adhesion,
(4) an anisotropic shear strength, and
(5) a tensile stress in the down-web direction that is at least 2 times greater than the tensile stress in the cross-web direction for all elongations up to the break elongation.
The pressure-sensitive adhesive component should be hot-melt processable and meet the Dahlquist criteria as described in Handbook of Pressure-sensitive Adhesive Technology, Edited by D. Satas, pg. 172, (1989) at use temperatures. Typically, the pressure-sensitive adhesive component comprises 30-98 weight percent of the composition, preferably 40-95 weight percent and more preferably 60-95 weight percent. Furthermore, the pressure-sensitive adhesive component could be a single pressure-sensitive adhesive or the pressure-sensitive adhesive could be a mixture of several pressure-sensitive adhesives.
The thermoplastic material component is typically a high polymer that can soften when exposed to heat and can return to the solid state when cooled to room temperature. Useful thermoplastic materials are fiber formers and are essentially immiscible in the pressure-sensitive adhesive component at use temperature, although the thermoplastic may be miscible in the pressure-sensitive adhesive at processing temperatures. Typically, the thermoplastic material component comprises 2-70 weight percent, preferably 5-60 weight percent and more preferably 5-40 weight percent. Furthermore, the thermoplastic material component could be a single thermoplastic material or the thermoplastic material could be a mixture of several thermoplastic materials.
In another aspect, a melt process for blended pressure-sensitive adhesives is described. Both components are melt mixed in a vessel and formed into a blended pressure-sensitive adhesive composition. The forming step is either (1) extruding the melt blended components under shear and/or extensional flow conditions or (2) extruding and drawing the melt blend. The formed composition is then cooled.
Also provided are pressure-sensitive adhesive coated tapes and articles.